The present invention relates to a method and a circuit arrangement for detecting the readiness for operation of an oxygen measurement probe which is arranged in the exhaust duct of an internal combustion engine and together with a regulation device serves to regulate the preparation of the mixture for the internal combustion engine.
In order to obtain an exhaust which is as free of injurious substances as possible, control devices for internal combustion engines are known in which the oxygen content in the exhaust duct is measured and evaluated. For this purpose oxygen measuring probes, so-called lambda probes, are known which operate in accordance with the principle of ion conduction through a solid electrolyte as a result of a difference in oxygen partial pressure and which give off a voltage signal which corresponds to the oxygen partial pressure present in the exhaust gas and exhibits a sudden change in voltage upon transition from deficiency of oxygen to an excess of oxygen.
The internal resistance of the known oxygen measurement probes is, however, so great at low temperatures that the signal given off by the oxygen measurement probe upon a cold start and during the warm-up phase of the internal combustion engine cannot be evaluated. In known devices for regulating the preparation of the mixture therefore a control which is independent of the output signal of the oxygen measurement probe is provided up to the region of readiness for operation of the oxygen measurement probe. Only after the oxygen measurement probe has reached its readiness for operation is its output signal used for regulating the fuel-air ratio.
In a known method for monitoring the readiness for operation of an oxygen measurement probe, the oxygen measurement probe is acted on by a test voltage having a constant mean value of the voltage which can be produced by the oxygen measurement probe. The resultant voltage at the output of the probe is fed as control variable of the readiness for operation to two comparison devices for comparison with an upper and a lower probe value corresponding in each case to a minimum output voltage of the oxygen measurement probe, and a mixture control device is connected or disconnected via a timing member, in the place of the mixture regulating device, in accordance with the output signal of the comparison device.
In another known method for the monitoring of the readiness for operation of an oxygen measurement probe, in which a voltage resulting under the influence of the behavior of the probe is also scanned by two comparison circuits with threshold voltages and then evaluated, a constant reference voltage is connected in opposition for the detection of the probe internal resistance which characterizes the readiness of the probe, the threshold voltages of the comparison circuits connected in opposition to the output voltage produced hereby lie by predetermined difference values above and below the reference voltage, in which connection the output signals given off by the comparison circuits and corresponding to a total of three logical switch states are processed by a subsequent evaluation circuit for switching between regulation and control or vice versa.
It is an object of the present invention to indicate a method for the detecting of the readiness for operation of an oxygen measurement probe which can be carried out reliably with simple and economical circuit arrangements. Furthermore, by the invention it is to be made possible to adapt the recognition by simple means to the conditions prevailing at the time.